Add link to errata page for textbook

index.html

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 a real and difficult problem in artificial intelligence. 
 
 <p><b>Textbook:</b>
-Some readings will be drawn from <a href="http://www.statmt.org/book/">Statistical Machine Translation</a> 
+Some readings will be drawn from <a href="http://www.statmt.org/book/">Statistical Machine Translation</a> (<a href="http://statmt.org/book/errata.html">errata</a>) 
 by <a href="http://homepages.inf.ed.ac.uk/pkoehn/">Philipp Koehn</a> 
-(You can read it online through the <a href="https://catalyst.library.jhu.edu/catalog/bib_3522360">JHU library</a> or 
-or purchase from <a href="http://www.amazon.com/Statistical-Machine-Translation-Philipp-Koehn/dp/0521874157">Amazon</a>).
+You can read it online through the <a href="https://catalyst.library.jhu.edu/catalog/bib_3522360">JHU library</a> or 
+or purchase from <a href="http://www.amazon.com/Statistical-Machine-Translation-Philipp-Koehn/dp/0521874157">Amazon</a>.
 A more compact (but therefore less thorough) <a href="http://www.cs.jhu.edu/~alopez/papers/survey.pdf">survey</a> by 
 <a href="http://www.cs.jhu.edu/~alopez">Adam Lopez</a> is available for free. Note that the readings (and therefore the textbook) 
 aren't strictly required; what's required is that you understand the concepts and are able to apply them.</p>